This invention relates to systems and methods for controlling a brushless DC (direct-current) motor.
Brushless DC motors, also known as self-synchronous or electronically commutated motors, may be used in a variety of mechanical systems, including motor vehicles and aerospace systems. For example, brushless DC motors may be found in automotive engine cooling systems and in heating, ventilation and air-conditioning (HVAC) equipment. A brushless DC motor typically includes a plurality of windings (or coils) wound about a stator, and a plurality of permanent magnets mounted on a rotor. An electronic control system switches current in the stator windings in a process known as commutation. The control system senses the position of the rotor and applies a coordinated sequence of control signals to electronic switches (e.g., transistors) that control the flow of current through the stator windings. The sequential switching of current through the motor windings produces a magnetic flux that rotates the rotor. Drive currents may be applied to the motor windings continuously in a linear mode of operation, or discontinuously in a nonlinear mode of operation using, e.g., pulse width modulation (PWM) techniques.
In order to apply the proper drive currents to the stator windings, the position of the rotor with respect to the conducting (or active) stator windings must be known. Various techniques may be used to detect the position of the rotor. For example, some systems derive rotor position information from sensors (e.g., Hall-effect sensors and optical sensors) coupled to the motor shaft. Other systems derive rotor position information from the back electromotive force (EMF) voltages generated in the windings as the motor rotates.
The invention features systems and methods for controlling a polyphase brushless direct current (DC) motor having a plurality of phase windings energizable by timed application of drive voltages to a plurality of motor terminals resulting in back electromotive force (EMF) conditions at the motor terminals.
In one aspect, the invention features a system comprising a meter coupled to a motor terminal and adapted to produce a range of digital output values representative of a motor terminal signal from which a back EMF signal may be derived.
In another aspect, the invention features a system comprising a meter, and a controller coupled to the meter and operable to monitor the range of meter output values and to compute a motor commutation time based upon a monitored local minimum meter output value and a monitored local maximum meter output value.
The invention also features a system comprising a pulse width modulation (PWM) circuit, and a meter coupled to a motor terminal, synchronized with the PWM circuit and configured to produce an output representative of a motor terminal signal from which a back EMF signal may be derived.
Embodiments may include one or more of the following features.
The meter may include an analog-to-digital (A/D) converter. A buffer may be coupled between the motor terminal and the meter. The buffer preferably is configured to scale the terminal signal values to a selected range. The buffer may include a voltage divider.
The motor commutation time preferably is computed based upon an identified phase-to-phase zero crossing in a back EMF signal. The controller may be configured to store a local minimum meter output value and a local maximum meter output value every motor terminal driving cycle. The zero crossing preferably is identified when the meter output value is substantially equal to one-half of the difference between the local maximum meter output value and the local minimum meter output value.
When the meter is synchronized with a PWM circuit, the meter preferably is adapted to sample motor terminal voltages in a non-conducting motor phase at times when each conducting motor phase is being driven.
In another aspect, the invention features a method of controlling a polyphase brushless direct current (DC) motor. In accordance with this inventive method, a range of signal values representative of a motor terminal signal from which a back EMF signal may be derived is monitored. Over the range of monitored signal values, a local maximum signal value and a local minimum signal value are stored. A motor commutation time is computed based upon the stored local maximum signal value and the stored local minimum value.
Among the advantages of the invention are the following.
The invention determines when zero crossings in the back EMF signals occur based upon direct measurements of the motor terminal signals. In this way, the invention avoids the sensors and complex circuitry, such as an ASIC or a plurality of comparators, that typically are used to determine when zero crossings occur. Furthermore, because the local maximum back EMF signal values and the local minimum back EMF signal values may be measured periodically (e.g., once every driving cycle), the invention readily accommodates component drifts and temperature variations.
Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims.